Distributed networked sensor systems are used in scientific, military, and law enforcement applications. By way of example, Unattended Ground Sensors (UGSs) have been considered as a low cost means to detect illegal border crossings along remote stretches of the border of the United States and other countries. Such illegal border crossings are associated not only with illegal immigration but also with drug trafficking and terrorist organizations. The basic requirements for UGS networks include long term unattended operation (multi-year), covert deployment, wide area of coverage, long (e.g., up to 50) mile communications range, low false alarm rate, and cost effectiveness.
State-of-the-art UGS systems currently have limited applications for border monitoring as these existing systems do not offer wide area coverage. Existing UGS systems generally have a few sensor modules that are placed in an area of interest (such as trail). The sensor modules are often plugged into a data processing unit which has radio capability. A few of these data processing units are deployed in a relatively small area and report either to a satellite relay module or to a radio relay station. The radios of relay-stations are active at all times and require a significant battery capacity for relatively short duration deployments (2-3 months). The sensor modules and their data processing units use less power as they can wake-up, take a reading, and transmit the collected data to an always active relay station or satellite communications module. For border monitoring applications, it is desirable to place a much larger number of sensor modules (hundreds instead of 10-15) over wide areas of land to detect people and vehicles. Many of these areas of deployment are quite remote and therefore require relay stations to communicate with the command and control center.
The time for an UGS system to report a target to the command and control center is short. An average person moves at 5 mph. At this speed, border patrol authorities have about 12 minutes to apprehend the person to restrain his or her movement to within one mile of the border. This requires the person to be identified and classified and the collected information reviewed by a border patrol agent in minutes. It is thus important for the UGS system to be capable of reporting alarms and making the sensed data quickly.
In fighting drug trafficking activities, sensor deployment and redeployments need to be covert, as do the sensors themselves. As UGS are frequently deployed using intelligence gathered by field agents (suspected areas of operation), it is likely that the deployment area may be under surveillance by the adversary. Several examples can be cited where the adversary either removed the sensors shortly after deployment or, in one case, moved the sensors downstream so as not to tip off the law enforcement officers by deactivating the system. Based on these experiences, it is desirable to make the UGS easy to deploy covertly, difficult to tamper with after deployment, and difficult to see once deployed. Other desirable objectives for a UGS include: network scalability to accommodate a large number of sensors; low power operation for all field modules (including the current high power consuming relay stations); long range communication ability (e.g., anywhere from 1 to 50 miles); rapid alarming; covert deployment; and rapid and automated provision of the data to a decision maker.
The largest single missing element in current UGS systems is the lack of a commercially available low-power mesh networking solution for radios with multi-mile communications range. This missing element creates a network scalability issue and requires a mixture of high power consuming relay stations with low power sensor modules. As background, mesh networking allows sensor modules to act as data relays for other sensor modules, thereby eliminating the constraint that all the sensors must have line-of-sight communications to a base (gateway) node. Mesh networking can greatly simplify installation as the installer need only confirm that the sensor he or she is currently deploying has line of sight to at least one other sensor or a base node. Mesh networking also can increase reliability. If one link in the network drops out, the network can reform to get data back to the base. While a great deal of research has been done for mesh networking for battery-powered radios with short range communications (10 m to 200 m) such as “smart dust” and Zigbee, very little has been done for radios with multi-mile communications range as required by applications such as border monitoring. The little work that has been done in this area assumes unlimited power at many of the “routing” nodes, which, again, does not match well with many UGS applications.